The present invention relates to a MODEM for use with Data Terminal Equipment (DTE) such as a computer or a facsimile apparatus for data communication via a telephone line. Specifically, the invention relates to a MODEM which selects an appropriate modulating method from a plurality of modulating methods.
Description of the Prior Art
Several kinds of modulation methods are available in MODEMs. For example, frequency modulation, four-phase differential phase shift modulation, quadrature amplitude modulation, and phase amplitude modulation are available. Selection of these methods may depend, for example, on the speed of communication. Recently, many MODEMs, for example a MODEM incorporated in a facsimile apparatus, switch the modulation/demodulation mode as well as communication speed depending on the protocol. It is desirable to synchronize the timing of a mode switch between a transmitter and a receiver. Conventionally, DTE, the host apparatus of a MODEM, selects an operational mode. The MODEM follows an instruction from the DTE to switch to the appropriate operational mode.
The DTE is connected to the MODEM via a BUS. The CPU contained within the DTE distinguishes between transmitted/received data by reading/writing to the transmitting-data-register and receiving-data-register which are both housed in the MODEM. In addition, the CPU recognizes the present operational mode of the MODEM by reading the status-register, and switches the operational mode of the MODEM by writing a mode number into the mode register. The modulating and demodulating portion of the MODEM follows an instruction from the mode register, and reports the status of each operation to the DTE via the status-register.
Before the ITU (International Telecommunication Union) announced the V.34 Standard based on the ITU-T Recommendation, many MODEMs used in facsimile machines were of the half-duplex type, and timing was not available for transmitting a waveform simultaneously from the transmitter and receiver. Therefore, either the transmitter or the receiver transmitted a waveform. The sequence of signal-transmission was determined by a protocol such as the T.30 Standard.
An operation according to the T.30 Standard is explained below:
If the previous mode is the receive mode, and the content of the received data is determined to require the next mode to be the transmitting mode, the DTE decodes the received data and instructs the MODEM to switch the mode. Software processing in the DTE requires time, however, and produces a signal gap between the end of the receiving signal and the start of the subsequent transmission. In half duplex communication the signal gap length will not cause a disruption in communication.
On the other hand, when the communication is based on the full duplex mode of the V.34 Standard, signals are transmitted simultaneously from both sides by communicating through a control channel. Accordingly, when the previous mode was the full-duplex mode of the control channel, and the next mode is to be the receiving mode of a main channel, the DTE determines the control channel data and gives a mode switching command to the MODEM. A problem occurs in the above case: The timing of the mode switch depends on an instruction from the DTE. If processing the transmitted/received data by the DTE takes time, the MODEM cannot receive the main channel, because of a delay in switching from receiving of control channel data of the previous mode to receiving of main channel data of the following mode. When a high speed MODEM is employed, processing data volume increases, whereby these types of problems frequently occur.
When a V.34 Standard half-duplex MODEM is used, the main channel of one-way communication and the control channel of two way simultaneous communication are alternately switched. In this case, if a signal gap is greater than a specific period (0.1 seconds), an echo-suppresser is recovered, whereby the control channel cannot complete the two-way simultaneous communication, and the communication is unable to recover. In order to prevent this problem, the V.34 Standard specifies the signal gap to be within 70 ms .+-.5 ms.
Therefore, a mechanism which shortens the processing time is required in an interface between the MODEM and the DTE. However, if the processing time can be shortened at the DTE side and switch timing can be accomplished in a shorter period in proportion to the signal gap length, monitoring the signal gap length desirably requires approximately .+-.5 ms accuracy. At the DTE side, software for monitoring the MODEM status thus desirably requires top priority, whereby the system design of the DTE becomes difficult.